In electrospray ionization, ions are formed in an ionizing region which is generally at atmospheric pressure and are drawn into a low pressure region through an orifice or tube where they undergo a viscous jet flow expansion. A skimmer is generally located adjacent the tube orifice and passes ions flowing along the axis to a lower pressure analyzing region. Electrospray ionizing assemblies are described in U.S. Pat. Nos. 4,121,099; 4,861,988; 4,137,750; 4,531,056 and 4,542,293. U.S. Pat. Nos. 4,861,988 and 4,121,099 disclose focusing of the ions after they enter the low pressure region. In U.S. Pat. No. 4,861,988 the ions are focused by a cylindrical member located in the analyzing region and following skimmers. In U.S. Pat. No. 4,121,099 a conical member spaced from an aperture which communicates between the ionization region and a lower pressure region.
In U.S. Pat. No. 4,542,293 there is described the use of a capillary tube made of an electrical insulator for conducting ions in the ionizing electrospray region at atmospheric pressure and a low pressure region. A glass or quartz capillary is suitable. Ions and gas are caused to flow from the ionization region through the tube into the low pressure region where free jet expansion occurs. A conductive coating is formed on the ends of the insulating tube and a voltage is applied thereacross to accelerate ions which flow through the tube. A conducting skimmer is disposed adjacent the end of the tube and is maintained at a voltage which causes further acceleration of the ions through and into a lower pressure region including focusing lenses and analyzing apparatus.
When the gas with entrained ions undergoes free expansion, the center of the flow is skimmed by the skimmer. However, a high percentage of the ions escape and are not captured by the skimmer.